In image-forming devices such as electrophotographic devices, electrostatic recording devices, electrostatic printing devices and so on, a method of forming a desired image by developing an electrostatic latent image formed on a photosensitive member with a toner is widely employed. Such a method is applied to copying machines, printers, facsimile machines, multi function products thereof and so on.
For example, generally an electrophotographic device using electrophotography uniformly charges the surface of a photosensitive member formed of photoconductive material with any of various means, and then, an electrostatic latent image is formed on the photosensitive member. Next, the electrostatic latent image is developed using a toner. After transferring an image of the toner on a recording material such as paper or the like, the image of the toner is fixed by heating or the like. Thus, a copy is obtained.
As toners used for the image forming device, toners, in which external additives such as inorganic particles and organic particles having smaller particle diameter than that of colored resin particles (toner particles) are attached (externally added) on the surface of the toner particles, are generally used for the purpose of improving functions such as charging ability and flowability of the toner, thereby obtaining desired printing performance.
However, if the toner using a conventional external additive is used, charge change is more likely to occur when initial printing is performed under a severe environment such as high temperature and high humidity (H/H), compared to the case where the initial printing is performed under a normal temperature and normal humidity environment (N/N), and a function of the external additive (a function to impart charge stability and flowability to the toner) cannot be maintained. Therefore, it has been problems that an initial charging speed decreases, a deterioration in image quality due to initial fog or the like occurs, and adverse effect on initial printing performance is caused.
In addition, in the process of continuous printing of a large number of prints, defects such that particles of the external additive are buried on and/or released (detached) from the surface of toner particles are likely to occur due to mechanical stress (increase in the number of contact of the toner particles by agitation or the like) in a development device. Therefore, it has been problems that reproducibility of thin lines decreases and a deterioration in image quality due to fog or the like occurs, and adverse effect on printing durability is caused.
Accordingly, development of a toner is demanded, wherein, in the stage of initial printing, a suitable initial charging speed can be exhibited without being affected by the usage environment, and in the process of continuous printing of a large number of prints, defects such as burial and/or release of the particles of the external additive are unlikely to occur under the mechanical stress in the development device, the toner can maintain the state in which the particles of the external additive are suitably attached, and stable charging ability (charge stability) can be exhibited.
For example, Patent Literature 1 discloses a toner obtained by using highly-hydrophobic heat treated spherical sol-gel silica fine particles having an average primary particle diameter of 0.01 to 5 μm as an external additive, in which heat treated spherical sol-gel silica fine particles are hydrophobized with a silane compound, for the purpose of improving flowability of a toner, caking resistance, fixability, cleaning property and environmental stability of a charge amount.
Patent Literature 2 discloses a toner obtained by using surface modified silica fine powders as an external additive, in which silica fine powders are surface-treated with alkylalkoxysilane having an alkyl group being a hexyl group of 6 carbons, or an alkyl group of less than 6 carbons for the purpose of improving flowability and durability of a toner, and further improving cleaning property by inhibiting the generation of filming and fog.
Patent Literature 3 discloses a toner obtained by using fatty acid alkali metal salt particles or fatty acid alkaline earth metal salt particles having a number average primary particle diameter of 0.1 to 1 μm, and two kinds of silica particles each having a different particle diameter as external additives, for the purpose of having excellent charge stability of toners upon replenishment of toners and improving initial printing performance and printing durability.